1. Field of the Invention
The present invention relates to a dummy light-exposed substrate and a method of manufacturing the same used in an immersion exposure apparatus, an immersion exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
An exposure apparatus which reduces and projects a pattern formed on an original (also called a mask or a reticle) onto a substrate by a projection optical system to expose the substrate to light has been conventionally used in the process of manufacturing a semiconductor device, having an ultrafine pattern formed, such as an LSI.
Along with an improvement in the integration density of semiconductor devices, further advances in micropatterning are required, leading to a growing demand for a higher resolving power of the exposure apparatus along with the development of the resist process.
To improve the resolving power of the exposure apparatus, there are an approach of shortening the wavelength of the exposure light, and an approach of increasing the numerical aperture (NA) of the projection optical system. As the resolving power is improved in this way, the depth of focus of the projection optical system decreases, so it is important to improve the focus accuracy of focusing the substrate (wafer) on the image plane of the projection optical system.
Also, one of evaluation items for the exposure apparatus is the alignment accuracy of precisely overlaying various patterns, and a factor which adversely affects the alignment accuracy is a magnification error of the projection optical system. The patterns of semiconductor devices are increasingly becoming finer each year. To keep up with this trend, a need for improving the alignment accuracy is soaring. It is therefore very important to maintain the projection optical system at a predetermined magnification throughout the light exposure process.
The projection optical system naturally absorbs a certain component of the exposure light energy, thus changing the temperatures of optical elements which constitute the projection optical system. Changes in the temperatures of the optical elements vary the optical characteristics, such as the refractive indices, of the optical elements.
When the projection optical system continues to be irradiated with exposure light over a long period of time, the imaging performance (e.g., the focus, magnification, distortion aberration, astigmatism, and other wavefront aberrations) of the projection optical system fluctuates. This may result in the generation of aberrations by amounts that are non-negligible from the viewpoint of ensuring high focus and alignment accuracies.
Under the circumstances, a method of correcting fluctuations in imaging characteristics which depend on the state in which the projection optical system is irradiated with exposure light has been proposed. In one example, the amounts of fluctuations in imaging characteristics which depend on the state in which the projection optical system is irradiated with exposure light are calculated using a model equation which uses, for example, the exposure light amount, light exposure time, and non-light-exposure time as variables, and the fluctuations in the imaging characteristics of the projection optical system are corrected based on the calculation results.
A model equation as described above can serve to correct fluctuations in the imaging characteristics of the projection optical system as long as the model equation has a coefficient for each imaging characteristic of the projection optical system and the coefficients are measured experimentally.
In the experiment for measuring the coefficients, there is no need to transfer a certain pattern on a substrate coated with a photoresist. For this reason, in the experiment for measuring the coefficients, a dummy light-exposed substrate placed on the substrate stage is irradiated with light in place of a substrate coated with a photoresist. Such light irradiation for a purpose other than light exposure as the original purpose can be called dummy light-exposure.
Nowadays, in order to achieve an actual increase in the numerical aperture (NA) of the projection optical system, an immersion exposure apparatus in which a liquid film is formed in the space between the projection optical system and the substrate and which exposes the substrate to light via the projection optical system and the liquid film has been realized. In the immersion exposure apparatus, the dummy light-exposed substrate must undergo dummy light-exposure while it is set in an immersion environment as well as the substrate used in the process. It is therefore convenient for the dummy light-exposed substrate to have a liquid repellent surface structure as well as the substrate used in the process. However, if the liquid repellent region is made of an organic substance, this substance decomposes upon being irradiated with excimer laser light while it is in contact with the liquid film. This makes it difficult to maintain the liquid repellency of the dummy light-exposed substrate in dummy light-exposure which requires a long period of time or a large number of times of light exposure.
A general inorganic substance which is expected to have resistance against excimer laser light is lyophilic. If the surface of the dummy light-exposed substrate is made of a lyophilic inorganic substance, the liquid film may stay behind on the surface, or the liquid may scatter depending on the leaving speed and/or transport speed of the dummy light-exposed substrate from the space beneath the projection optical system.